- Increase Redis cache TTL from 30 minutes to 24 hours
- Add stale-while-revalidate: serve stale cache (>4h) immediately while
repopulating in background with SETNX lock to prevent concurrent rebuilds
- Add in-memory frontend LRU cache (5 entries) so repeat filter visits
are instant without network requests
- Invalidate frontend cache on listing refresh and task completion
- Add unit tests for get_cache_age, is_cache_stale, acquire_repopulation_lock
